1. Field of the Invention
The invention relates to a liquid crystal display (LCD), and more particularly, to an apparatus and method for generating bias voltages for an LCD driver.
2. Description of Background Art
Liquid crystal displays (LCDs) are digital displays widely used in digital watches, calculators, handheld game machines, and various other electronic appliances. The circuit structure of a typical LCD device is shown in FIG. 1, in which an LCD driver 10 in conjunction with a voltage divider 20 are used to drive an LCD panel 30. In practice, the LCD driver 10 and the voltage divider 20 are implemented in an integrated circuit (IC) as indicated by the dashed box 1. The voltage divider 20 consists of a number of resistors R that divide an external voltage Vcc into bias voltages V.sub.a, V.sub.b, V.sub.c, V.sub.d, and V.sub.e. These bias voltages are applied to and drive the LCD driver 10 to generate a plurality of LCD driving signals, including common signals, via the COM1-COM8 lines and segment signals via the SEG1-SEG40 lines.
In the voltage divider 20, the plurality of resistors R constitute a DC current path through which a DC current I.sub.d flows. These resistors are provided with high resistances, such as 100 k.OMEGA. or 200 k.OMEGA. so as to minimize the current I.sub.d flowing through the DC current path. A drawback to the use of high resistance resistors is that the resulting driving current used to actuate the LCD driver for switching of the LCD driving signals may be insufficient. To cope with this problem, a conventional method is to provide a corresponding number of capacitors C connected externally via I/O pins on the IC 1 to the voltage divider 20. These capacitors C are used for voltage stabilization of the circuit so as to supply sufficient actuating current I.sub.t to the LCD driver for switching of the LCD driving signals.
ICs based on the foregoing circuit architecture for generating bias voltages include MSM5238GS, MSM5259GS, and MSM5278 which are manufactured by the OKI Semiconductor Corporation. However, providing the externally connected capacitors has two drawbacks. First, for low-cost LCD handheld game machines, the provision of these externally connected capacitors and the corresponding I/O pins significantly increases manufacturing cost; and second, the increased number of I/O pins on the IC would cause the size of the chip to be larger than it would be otherwise.
Two methods have been used to eliminate the foregoing two drawbacks. The first method is to avoid using the capacitors and reduce the resistance values of the resistors R so as to provide a larger DC current I.sub.d. However, this causes a large leakage current. For example, assuming in the circuit of FIG. 1 that R=100 k.OMEGA. and Vcc=5 volts, then I.sub.d =5/(100 k.times.5)=10 .mu.A. However, if R is reduced to 15 k.OMEGA., then I.sub.d =5 V/(15 k.OMEGA..times.5)=67 .mu.A. Since the IC needs only a small amount of current for operation, such a large current of 67 .mu.A would cause much of the electrical power to be wasted. The second method is to provide built-in capacitors in the IC. However, this increases the area of the chip and such capacitors would be very low in capacitance, several orders from the desired level.